Madrid, Valencia and other cities are being targeted to run similar campaigns.

Campaigners say that with 20% of Spaniards professing they do not believe in God, it is time atheism becomes a visible phenomenon.

"It is time for non-believers to make themselves seen and display their pride in their own convictions," said the Catalan Atheists group.

The campaign has provoked a reaction from the Catholic archbishopric of Barcelona. "Faith in God is not a source of worry, nor is it an obstacle for enjoying life," it said in a statement.

"It is an attack on all religions," said Javier Maria Perez-Roldan of the church's Tomas Moro centre, blaming the socialist government for the privately funded campaign. "The government has created an atmosphere of belligerence."

The go-ahead for campaigns in other Spanish cities may depend on the political colour of the city halls that own, or co-own, the municipal buses.

Conservative officials in Madrid and Valencia are coming under pressure from Catholic lobbyists to prevent the advertisements circulating around their streets.

The spread of the Spanish campaign will also depend on how much funding atheist groups can raise.

Donations have been flooding into the fund opened by the Catalan Atheists. "We raised a thousand euros in the first day," the group said. "At this rate we will be able to take the campaign to Madrid as well."

Negotiations to get the advertisements on to Madrid's buses are reportedly set to start today.

The atheist campaign comes as Spain's Catholic church becomes increasingly involved in political campaigning.

It has fought against laws passed by prime minister José Luis Rodríguez Zapatero's socialist government allowing gay marriage, simplifying divorce and reducing the importance of religious instruction in the school timetable. It is campaigning against changes to Spain's abortion laws.

Although church and state are nominally separate in Spain, the Catholic church receives funding from the government.